A cellular radio system comprises fixed base stations, each having a certain coverage area, and terminals that can move with respect to the base stations and their coverage areas. The coverage areas are also called cells. This patent application uses mobile phone as an example of a terminal. When a mobile phone is switched on it tries in a way or another to find the strongest base station signal and to register to the so-called location area (LA) represented by the base station in question. Registering means that the mobile phone informs via the base station the mobile telephone network that it can be paged through the location area to which the base station in question belongs. In the idle mode the mobile phone regularly receives messages sent by the base station in order to detect paging messages indicating incoming calls and other messages sent to it. At the same time the mobile phone observes the power of the signals of other nearby base stations in order to be able to quickly change base stations when necessary.
In prior-art cellular radio systems, base stations are substantially alike as regards communications characteristics; in other words a mobile phone receives the same kind of service from the network regardless of the cell in which it is operating. An exception to this rule is made by certain cell prioritising arrangements with which mobile phones are usually directed to operate in cells belonging to a special home area or to avoid certain so-called handover cells. In the home area arrangement a mobile phone is assigned a fixed geographical home area by a contract between the user and the operator maintaining the cellular radio system. When the mobile phone is operating in a home area cell, it is allowed a discount for the normal call tariff or other advantages.
In prior-art cellular radio systems, also the terminals are very much alike as regards communications characteristics. A system usually specifies only one kind of air interface the specifications of which relate to transmission and reception timing available frequencies, and the data transfer rate, or bit rate, as it is called.
Cell selection functions in the GSM (Global System for Mobile Telecommunications) and its extension DCS 1800 (Digital Communications System at 1800 MHz) are specified in standards ETS 300 535 (GSM 03.22) and ETS 300 578 (GSM 05.08) by the EBU (European Broadcasting Union) and ETSI (European Telecommunications Standards Institute). On a general level the same approach can also be applied in other digital cellular radio systems. Cell selection is also discussed in Finnish patent application FI-970855 (Nokia Mobile Phones Oy). Cell selection may also be called base station selection since the radio communications of each cell are handled by a certain base station.
In studies of a new kind of mobile communications system called the third-generation mobile communications system it has been discovered that known methods cannot always control the cell selection in the best possible manner. Third-generation systems include the UMTS (Universal Mobile Telecommunication System) and FPLMTS/IMT-2000 (Future Public Land Mobile Telecommunication System/International Mobile Telecommunications at 2000 megahertz). It is planned that in the new systems mobile terminals will have 3 to 4 operating environments, which can be characterised according to the cell size and the mode of movement or location of the terminal. The operating environments are: vehicular environment (macro cells, kilometers), pedestrian environment (microcells, hundreds of meters), office environment (nano and/or picocells, meters) and possibly satellite environment (megacells, hundreds of kilometers). The latter means that the base station communicating with the terminal is located on a satellite or that the base station is terrestrial but there is a satellite link between it and the terminals. In different operating environments the cell size and terminal mobility provide different possibilities of implementing error-free communications which is reflected in the bit rates. The office environment may offer communications at up to 2 Mbit/s, whereas in the vehicular environment you'll probably have to be satisfied with a maximum of 144 kbit/s.
If every terminal always tries to choose the cell offering the fastest possible communications, it is likely that terminals are directed to "fast" cells in such great quantities that their simultaneous random access requests cause a collision and a connection failure. When a connection to a first base station has been established and the terminal and first base station have agreed on a certain service level, the connection can be handed over to a base station which has a cell that is at least partly overlapping with the cell of the first base station and in which the capacity situation better allows the connection in question. However, handovers increase the signalling load of the network, so it would be better if the connection could be established with a suitable base station from the start.
From patent document U.S. Pat. No. 4,916,728 (Blair) there is known a method in which a mobile phone can operate in networks of several operators. In order to be able to select the most advantageous network, a mobile phone goes through several reception frequencies, decodes from the signals it has received the system identification (SID) codes, and tunes in at the frequency the SID code which indicates the most advantageous operator. Data for the different operators have been stored in the mobile phone's memory, so in this arrangement various mobile phones respond to the information sent by the base stations in various ways. However, as all base stations in the network of a given operator send out the same SID code, mobile phones in this method cannot be made to operate in different ways except for the selection of the operator.
PCT patent document WO-95/24809 (Motorola Inc.) deals with a system in which a switching centre uses an identifier sent by a mobile terminal to decide whether the mobile terminal is entitled to a certain service in a given area. If there are geographical and/or mobile phone specific restrictions on the service, the switching centre may either refuse to offer any services to the mobile phone in that area or allow the use of a specific service, such as the data transfer service, for example. However, in order to change the supply of allowed services, the mobile phone has to be moved since the restrictions are always the same in a certain area. So, in this method it is not possible to affect the cell selection or re-selection when the mobile phone or other terminal of a cellular radio system is stationary.
What is known from Finnish patent application FI-952965 and from the corresponding European patent document EP-749 254 Al (Nokia Mobile Phones Oy) is a multiple-level home area pricing method for a mobile phone of a cellular radio system wherein a binary character sequence is stored in the mobile phone. Each base station sends at regular intervals a special binary identification code of its own and the mobile phone uses said stored binary character sequence as a mask to select from the character sequence sent by a base station certain bits for a logic operation. If said logic operation produces the right result, the mobile phone concludes that it is located in the home area or in another area where a certain territorial service is enabled. Using different logic operations it is possible to form several separate or mutually hierarchical areas in which the mobile phone is given different services by the cellular radio system. Again, this is a method that is not very suitable for cell prioritisation because the services are territorial and the service supply changes only when the mobile phone moves.
In addition to the methods mentioned above several methods and systems are known in which a mobile phone or other cellular radio system terminal can detect whether or not it is operating in a device-specifically prioritised cell and inform the user about the fact. However, no prior-art system is able to direct terminals to use a suitable cell if there are available cells that have different capacities.